In accordance with miniaturization of a semiconductor device in recent years, interwiring capacitance of a wiring layer, especially interwiring capacitance of an isolayer tends to increase. When interwiring capacitance increases, parasitic capacitance of a circuit increases, which leads to a decrease in operating speed of a semiconductor device. A semiconductor device is known which has a hollow wiring structure provided with an air gap between wirings in order to reduce the interwiring capacitance.
A method of forming a hollow wiring structure is suggested in which an opening is provided in an insulating film formed on an interlayer insulating film having a wiring formed therein, and after forming an air gap by eliminating the gasified interwiring film through the opening, an insulating film is deposited on the opening for the sealing. This method, for example, is disclosed in JP-A-5-21617.
According to the method described in JP-A-5-21617, by sealing the air gap with the insulating film, it is possible to prevent water or a material of an upper insulating member from entering inside of the air gap, thereby suppressing deterioration in operational reliability of a semiconductor device and an increase in electrical capacitance. However, in practice, it is not possible to completely block the opening due to a problem such as embedding characteristics of an insulating film, etc., which may result in letting water or an insulating material enter in the air gap, or which may result in that an insulating film itself enters in the air gap.
In addition, as described in JP-A-5-21617, when a width of the opening is narrowed by a method such as forming an insulating film sidewall on a side face of the opening for facilitating the blocking of the opening, since the width of the opening is narrow, a removal of the interwiring film, i.e., a formation of an air gap may become difficult.